Green’s Dictionary of Slang

to interrogate forcefully and/or violently; thus third-degreeing n.

1914		Bulletin (Sydney) 2 July 36/3: It isn’t necessary to ‘third degree’ some of these suburbanites to find out what they are thinking about.
1926		J. Black You Can’t Win (2000) 207: I wasn’t taken out of my cell and ‘sweated’ or third-degreed, or beaten up.
1936		‘Banjo’ Paterson Shearer’s Colt 62: I expect they are going to put the third degree on his mate and see if he’ll squeal before they take Jimmy in.
1948		C. Day Lewis Otterbury Incident 116: ‘We’ll put the Prune to the question’ [...] ‘Yeah. Third-degree him,’ said Nick.
1953		T. Runyon In For Life 192: He wasn’t gently paving the way for a little third-degreeing, was he?
1967		in S. Harris Hellhole 153: Cindy explains the point well in her story of her ‘third-degreeing’ by a certain officer.
1975		N. De Mille Smack Man (1991) 128: They’re really third-degreeing every pusher and junkie they get their hands on.